4See the appendix for definitions of the
various types of degradation.

Change in soil quality over time can be a complex phenomenon.
Quality can vary across sites, soil types, and production systems. Furthermore,
soil quality is only one of many variables influencing agricultural yield, which
is, in turn, only one of many factors influencing food consumption, food
availability, and farm income. This complicates the evaluation and
interpretation of the effects of soil degradation and the design of appropriate
policies in response.

Box - Agricultural Productivity and Soil Quality

Soil quality is the inherent capability of the soil to perform a
range of productive, environmental, and habitat functions. This study is
concerned mainly with the soils productive function, hence it is important
that the definitions of productivity used below in relation to soil quality are
clear.

Diverse definitions of productivity have created
some confusion. In this paper, the term potential soil productivity
is used to refer to the potential of the soil system to accumulate energy
in the form of vegetation (following Tengberg and Stocking 1997, 4), controlling
for the use of other inputs. Soil productivity is used to refer to
the actual yield of usable vegetation, also controlling for input use.
Agricultural productivity refers to the relationship between the
average or real output of economically usable products divided by an index of
all fixed and variable inputs. Because economists conventionally have analyzed
land productivity simply as total output divided by land area
(assumed to be a fixed factor), soil quality has not been considered. Yet
measures of change in total factor productivity over time that do
not include soil quality are likely to overestimate the contribution of other
factors. On the other hand, the effect of soil quality change on agricultural
productivity is limited by its importance as a productive factor relative to
other factors, and the degree of complementarity and substitutability between
soil quality and other factors and inputs. Soil quality contributes relatively
more to agricultural productivity in low-input production systems.

Vulnerability of Soils to Degradation

The widespread tendency to minimize the importance of soil
quality for agriculture stems in part from the experience of temperate
agriculture. The most productive temperate soils are geologically
new. A result of glaciation in the last Ice Age, these soils are
both fertile and relatively resistant to degradation. By contrast, though some
tropical highland soils are also new, formed through the deposition
of volcanic materials from old eruptions, most are of infertile parent material
or have been highly weathered over the millenia, resulting in the leaching of
soluble nutrients from soils and acidification. The higher temperatures, greater
high and low extremes of rainfall, and greater rainfall intensity typical of the
tropics subject soils in most developing countries to significant risk of
climate-induced degradation.

Indeed, only a third of all rainfed, cultivable area in
developing countries (excluding China, for which data were not available) is
free of major soil-related constraints that limit production (Table 1). The 10
percent of land in steep slopes is especially prone to erosion, as are shallow
soils; the extensive areas with low natural fertility require active nutrient
replenishment and supplementation to sustain even moderate yields over time; and
sandy soils require careful management to retain water. Chemical soil
constraints are also widespread: 36 percent of tropical soils have low nutrient
status; one-third have sufficiently acid conditions for soluble aluminum to be
toxic for most crops (acidity is exacerbated by inorganic fertilizer
application); 22 percent are tropical clays that fix phosphorus; 5 percent have
critically low cation exchange capacity; and some are saline or alkaline
(Sanchez and Logan 1992, cited in Tengberg and Stocking 1997, 9-10).

Poor land husbandry can have quite different long-term effects
on different types of soils, and costs of and returns to soil improvement can
vary substantially, depending upon soil resilience (the resistance to
degradation) and soil sensitivity (the degree to which soils degrade when
subjected to degradation processes). For example, ferralsols, which have low
available nutrient supplies, strong acidity, low available phosphorus, no
reserves of weatherable minerals, and easily lost topsoil organic matter,
demonstrate low resilience and moderate sensitivity to water erosion. Even with
good soil cover, yields decline rapidly without a combination of structures and
biological measures to control erosion. By contrast, luvisols, with moderate
nutrient levels, low-to-moderate organic matter content, and weak topsoil
structure prone to crusting, have moderate resilience and low-to-moderate
sensitivity. Maintaining their productivity requires both tillage practices that
maximize surface water infiltration and biological measures that maintain soil
cover (Tengberg and Stocking 1997; see Figure 1). While some soils, like
alfisols, can be maintained for a long time with only inorganic fertilizer
application (if farmers make sure that they do not crust), Luvisols require
complementary use of organic inputs because they are low in organic matter to
begin with (Swift 1997).

Table 1 - Share of land with terrain and soil constraints in
total rainfed land with crop production potential

Constraint

Sub-Saharan Africa

Latin America and the Caribbean

Near East/North Africa

East Asia (excluding China)

South Asia

Developing countries (excluding China)

(percent)

Steep slopes (16-45 percent)

11

6

24

13

19

10

Shallow soils (<50 centimeters)

1

10

4

1

1

1

Low natural fertility

42

46

1

28

4

38

Poor soil drainage

15

28

2

26

11

20

Sandy or stony soils

36

15

17

11

11

23

Salinity, sodicity, or excess of gypsum

1

2

3

1

2

1

Total land with crop production potential affected by one or
more constraintsa

An assessment of the productivity-related economic effects of
soil degradation that is relevant to policy-making first requires estimates of
the changes over time of the type, scale, and rate of physical soil quality at a
subregional or higher scale. These changes must then be linked to consequent
changes in agricultural yield or production costs, and these, in turn, to
resulting changes in consumption, market supply, farm income or economic growth,
and the long-term value of the resource base.

Assessing Soil Quality Change Over Time

Methods for soil quality assessment were developed mainly for
use at the plot level, and are problematic to scale up, even when substantial
plot-level data are available (Halverson, Smith, and Papendick 1997). No
developing country has in place a national monitoring system for soil quality.
Researchers trying to assess soil quality change above the plot level, have used
approximate measures, including

· Consultation with
experts, long familiar with particular regions, who provide a ranking or
qualitative assessment of the scale and processes of degradation within the
region, according to agreed-upon criteria (see, for example, Oldeman, Hakkeling,
and Sombroek 1991);

· Review and comparative
evaluation of published studies on degradation from many different sites within
a region (see, for example, Lal 1995; Dregne and Chou 1992);

· Extrapolation of the results
of case studies, field experiments, and other micro- or watershed-level data to
the national level (see, for example, cases in B996); and

· Estimates constructed from
examination of secondary data on land use change, representative ecological
conditions, and so on (see, for example, Rozanov, Targulian, and Orlov
1990).

Assessing the Effects on Agricultural Productivity

The effects of soil degradation on agricultural productivity
(see box) vary with the type of soil, crop, degradation, and initial soil
conditions, and may not be linear. Lower potential production due to degradation
may not show up in intensive, high-input systems until yields are approaching
their ceiling. Reduced efficiency of inputs (fertilizer, water, biocides, labor)
could show up in higher production costs rather than lower yields.

Effects on productivity are most commonly estimated using
coefficients based on plot-level experimental trials or cross-sectional farm
surveys. Many researchers estimate production effects using the Universal Soil
Loss Equation.5 Since trial and survey data are unavailable for a
number of soils and degradation processes, studies often base assumptions about
aggregate physical yield effects on degradation-yield relationships taken from
the literature or estimated by soil experts. Few studies use historical
time-series data on yield and production cost; even fewer attribute yield or
cost change to soil quality change, controlling for other variables.

5 The Universal Soil Loss Equation (USLE)
was developed in the 1970s to estimate erosion risks and levels in temperate
agriculture, but it has been adapted for tropical conditions. The USLE equation
is A = R*K*L*S*C*P, where A = long-standing average annual soil erosion in
metric tons/hectare; R = rainfall erosive factor (which depends on the
frequency, quantity, seasonal distribution, and kinetic energy of heavy
rainfall); K = soil credibility factor (dependent on soil type); L =
slope-length factor; S = slope steepness factor; C = farming practice and
crop-type factor (dependent on the stage of cultivation and the cover by crops,
other vegetation, or residues); and P =- soil conservation measures
(which depend on farm management practices). The USLE was developed and further
refined for use at the farm-plot level, but it has been widely applied (and some
would say, misapplied) at the landscape and even national levels to estimate
erosion (Wischmeier and Smith 1978).

Most research methods provide only a rough estimate of the
nature and relative importance of degradation across large areas, though a few
valuable studies disaggregate by type of soil, topography, location, crop, or
farm household.

Indicators of Economic Impact

Many different indicators have been used in research on the
economic effects of soil degradation. Welfare effects have been measured by
changes in the number of food-insecure households or malnourished children; the
amount of food consumed from farm production; the level of rural household
income or consumption; the degree of community- level food self-sufficiency; and
the rates of migration. Effects on agricultural supply have been measured by
changes in average crop yields or aggregate crop production, aggregate market
supply, export or import levels, and level and variability of crop prices.
Economic losses have been assessed by comparing the value of lost production,
the value of inputs needed to compensate for lost nutrients, or current or
discounted future income streams to farm income, national income, or economic
growth rates, or by measuring changes in input efficiency. Effects on national
wealth have been measured only by changes in the aggregate amount or quality of
agricultural land (Scherr 1997a).

Evolution of Methods for Impact Assessment

Studies of the productivity-related economic effects of soil
degradation can be divided into three periods. Those published in the late 1970s
and 1980s were intended mainly to draw public attention to the issue. They used
rather simplistic approaches, calculating gross aggregate effects of soil
erosion on agricultural lands (assuming little use of conservation practices)
and resulting gross economic losses.

Global and regional analyses published in the early 1990s were
more systematically designed and reflective of broad field experience. They
relied mainly on secondary data, literature reviews, and surveys of regional
soil experts, and used fairly simple economic models, if any. National and
sub-national studies used similar methods, but with more disaggregated data, to
construct models that measured impact. Typically in the early 1990s, the
economic impact of degradation was measured in terms of the value of lost
yields, the value of plant nutrients lost through erosion, or the costs of soil
rehabilitation. These changes were valued at market prices. The approaches of
this period have been criticized for their degree of aggregation, simplistic
assumptions about degradation-production relationships, failure to examine
least-cost alternatives to rehabilitation, and failure to consider likely farmer
or market responses to supply or cost shifts. Since the mid-1990s a third
generation of studies has used more sophisticated models and methods for
collecting and analyzing data to disentangle causal relationships and explore
variation in soil conditions and management (see, for example, Enters 1998).
Many projects have begun to collect primary data from representative soil, farm,
or village units in order to develop more reliable biophysical yield models for
different types of environments, degradation, and soil management. Research
increasingly focuses on effects at the national and subnational levels, and this
allows for more policy-relevant analysis (Scherr 1997a).

Predicting Future Effects: Conceptual Challenges

Even with the best information on past and current trends, three
other central issues must be considered before predictions about future trends
regarding soil degradation can be made with any confidence:

(1) To what extent is soil degradation reversible at
an economically reasonable cost?;

(2) To what extent will farmers respond on their own to protect
or rehabilitate their soils?; and

(3) To what extent will structural change in agricultural
economies affect our reliance on currently degrading soil
resources?

Reversibility of Soil Degradation

Where soil degradation is reversible at low-to-moderate economic
cost (relative to agricultural product prices and land values), even significant
degradation may result in little long-term economic loss. Prevention is not
always cheaper than a cure. For example, farmers who cease to undertake
soil-protecting investments during prolonged periods of low food prices may
resume those practices when prices rise. Farmers also may mine soil nutrients
(soil capital) over a period of time in order to accumulate alternative forms of
more economically valuable capital, but subsequently use that capital to rebuild
soil resources. Land abandonment after prolonged soil degradation could serve to
keep the land fallow long enough for it to recover key long-term productive
attributes.

If, on the other hand, degradation through lack of proper soil
husbandry in the short term leads to permanent reductions in the soils
productive potential, strategies leading to degradation are less likely to be
economically justifiable. What constitutes irreversibility is a
matter of some debate among soil scientists due to inadequate research. Only
nutrient depletion and imbalance and surface sealing and crusting can be rapidly
and relatively cheaply reversed (Table 2). Many water, nutrient, and biological
problems in soils can be reversed over 5-10 years through soil-building
processes and field- or farm-scale investments and management changes. Some
types of physical and chemical degradation, such as terrain deformation and
salinization, are extremely difficult or costly to reverse. The feasibility and
cost of soil rehabilitation depend in part on soil type, production system, and
severity of degradation. For many soil types, little is known about the effects
of degradation or the thresholds for soil quality below which future investment
in restoration is uneconomic.

Farmer Response to Soil Degradation

Historical evidence suggests that a linear extrapolation of
current soil degradation trends will be a poor guide to future soil quality.
Farmers depend upon the land for their livelihood. It is uncommon for them to be
unaware of serious soil degradation unless they are recent immigrants to a new
agroecological zone, the process of degradation has not yet affected yields, or
its cause is invisible (acidification, for example). We should expect,
therefore, that farmers will respond to degradation with new land management or
investment if they perceive a net benefit from doing so and can acquire or
develop appropriate technology. Trajectory 1 in Figure 2 illustrates such a
process of innovation, in which increasing pressure on soil resources over time
initially leads to soil degradation, but farmers eventually respond by improving
soil management practices and making investments to restore, maintain, or even
ultimately improve the soils productive potential. Empirical
examples of such a process have been widely documented (Ruthenberg 1980;
Templeton and Scherr 1997; Tiffen, Mortimore, and Gichuki 1994).

Table 2 - Relative reversibility of soil-degradation
processes

Type of degradation

Degradation process

Largely reversible, low cost

Reversible, significant cost

Largely irreversible/very high cost

Physical

Clay pans, compactionzz zones

X

Surface sealing and crusting

X

Subsidence

X

Topsoil loss through wind or water erosion

X (if active deposition)

X

Terrain deformation (gully erosion, mass movement)

X

Waterlogging

Waterholding Reduced infiltration/impeded drainage

X

Reduced waterholding capacity

X

Aridification

X (farm scale)

X (landscape scale)

Chemical Organic matter loss

X

Nutrient depletion/leaching

X

X

Nutrient imbalance

X

Nutrient binding

X

Acidification

X (if liming feasible)

X

Alkalinization/salinization

X

Dystrification

X

Eutrophication

X

Biological

Reduced biological activity due to soil disturbance

X

Reduced biological activity due to agrochemical use

X

Pollution

Contamination

X

Pollution (accumulation of toxic substances)

X

Source: Informal consultation with tropical soil
experts and various texts on degradation.

Farmers respond not only by making major conservation
investments such as terrace construction on steep slopes, land-leveling in
irrigated areas, land drainage, and revegetation of denuded landscapes, but also
by using alternative crop mixes and cropping intensities; land-clearing and
fallow practices; spatial patterns and niches of crop production; tillage and
planting density and timing practices; agro forestry practices; vegetation
management outside crop fields; crop-residue management; livestock population,
species, and feeding practices; or farming implements. Farmers may modify the
layout of farm paths, fences, windbreaks, and other linear features or barriers
in order to affect soil and water movement (Scherr et al. 1996).

The conservation community has discovered that farmers
decisions about conservation practices and investments are inextricably linked
to production (Shaxson et al. 1997). If good land-husbandry practices are to be
widely adopted, they must not only replenish soil resources, but also contribute
to increased productivity and farm income in the short term (Sain and Barreto
1996; Partap and Watson 1994). Farmer willingness to invest in soil improvement
is closely associated with the overall economic profitability of farming and an
economic and policy environment that facilitates commercialization, reduces
price risks, increases access to infrastructure, increases security of land
access, and encourages technical innovation (see, for example, Clay, Reardon,
and Kangasniemi 1998; Shiferas and Holden 1997; Hopkins, Delgado, and Gruhn
1994).

When farmers fail to take action (trajectory 2 in Figure 2) or
delay taking action until significant, irreversible degradation has taken place
(trajectory 3), it usually means that they lack knowledge about effective means
for soil improvement; lack access to the farm resources, such as labor, capital,
or inputs, needed to make the improvements (a particular concern for the poor);
believe the economic contribution of the plot to their livelihood is marginal;
expect low economic returns from available options for soil improvement; or are
uncertain about reaping the longer-term benefits of soil improvement due to
tenure insecurity or price or climate risks (Scherr and Hazell 1994). Under
these conditions, targeted policy action is needed to slow or reverse soil
degradation. Policy intervention may also be desirable to accelerate farmer
response in situations where social benefits are greater than farmers
private benefits (trajectory 4 in Figure 2).

Figure 2 - Innovation in soil resource
management under population or market pressure

Note: t0 to t3 are time
periods. Trajectory 1 indicates a flexible and innovative response to
degradation by farmers. Trajectory 2 indicates a failure to take action.
Trajectory 3 indicates a delay in taking action until significant degradation
has occurred. Trajectory 4 indicates that policy intervention encouraged farmers
to respond sooner or more effectively than would otherwise have been expected on
the basis of their existing incentives.

The trajectories of soil degradation and improvement vary
considerably among different pathways of development. These variations result
from differences in the soil resource base, demographic patterns, market
integration, local institutions, and policy actions (Clay, Reardon, and
Kangasniemi 1998; Scherr et al. 1996). Judicious use can be made of limited
public investment resources to address soil degradation only if we are able to
better predict when and how farmers will respond to degradation and
intervention.

Structural Change in Agricultural Economy

Even if existing estimates of the economic effects of soil
degradation in recent decades are correct, they cannot necessarily be
extrapolated to 2020. There is no certainty that all of the developing
worlds soils currently under cultivation will constitute important
resources for agricultural production in the decades ahead. Structural changes
in global and national economies, trading patterns, and infrastructure
development may make some soil resources much more important than others.
Technological breakthroughs may make some problem soils much more
productive in the future, while unforeseen events may contaminate soils that are
most productive at present. Thus, evaluation of future threats of degradation
requires that we assess the likely future trends in the broader economy and
their implications for soil management. Some possible scenarios are presented in
Chapter 4. Past and present challenges are presented first, in Chapter
3.